Abstract

In biological systems, the coupling of nonlinear biochemical kinetics and molecular transport enables functional sensing and “signal” amplification across many length scales. Drawing on biological inspiration, we describe how artificial reaction-diffusion (RD) microsystems can provide a basis for sensing applications, capable of amplifying micro- and nanoscopic events into macroscopic visual readouts. The RD applications reviewed here are based on a novel experimental technique, WETS for Wet Stamping, which offers unprecedented control over RD processes in microscopic and complex geometries. It is discussed how RD can be used to sense subtle differences in the thickness and/or absorptivity of thin absorptive films, amplify macromolecular phase transitions, detect the presence and quality of self-assembledmonolayers, and provide dynamic spatiotemporal readouts of chemical “metabolites.”

Received 31 March 2006Accepted 30 June 2006Published online 27 September 2006

Lead Paragraph: The unique characteristic of nonlinear systems lies in their ability to amplify incoming signals. Although modern technology, especially optoelectronics, has capitalized on nonlinear amplification as a basis for such important devices as lasers,1 power and frequency amplifiers,2,3 optical parametric amplifiers,4 and doped fiber amplifiers,5 it has not been able to apply it as broadly and flexibly as biological systems do. Indeed, in biology the coupling between inherently nonlinear (bio)chemical kinetics and the transport of chemicals makes nonlinear amplification phenomena ubiquitous at virtually all length scales. On the level of macromolecules, various ultrasensitive protein/gene regulatory cascades6 play the role of developmental “programs” and amplify molecular events into spatial and/or temporal patterns up to cellular7,8 or even organismal9,10 scales. In humoral immune response, B lymphocytes recognize and respond to new antigens by amplifying the production of antibodies that ultimately help destroy the foreign invader.11 In collections of microorganisms, cAMP signaling between the individual members of an ensemble translates/amplifies into their collective behaviors and visual appearance.12,13 Finally, among large organisms, nonlinear predator-prey dynamics can cause amplification (or extinction) of the entire species.14 Our work draws inspiration from a particular mode of biological amplification, in which molecular or cellular events are transformed into visual color and/or pattern changes (Fig. 1). For example, chameleons change their skin color (e.g., to indicate reproductive desires, to provoke combat or submit) as a result of endocrinal processes15,16 leading to microscopic movements of light-reflecting cells called iridophores and the crystals within these cells. Similarly, squids respond to feelings of fear by becoming iridescent. In this case, stimulation mediated by acetylcholine17 causes either a gel-sol phase transition in platelets contained in iridophores or a change in the platelet viscosity and thickness, with thinner platelets scattering light of lower wavelengths.18 The objective of our research has been to develop artificial systems that would, in a very primitive sense, behave like chameleons or squids and dramatically change their macroscopic appearance in response to underlying microscopic/molecular properties or processes (“signals”). The practical rationale of this effort is that the visual readouts could be used to monitor and report the details of events occurring at much smaller scales—a capability that would be useful in a variety of sensory and detection applications.

Acknowledgments:

This work was supported by NSF Career Award No. CTS-0547633, Baxter Healthcare Research Award, 3M Nontenured Faculty Award, and the Camille and Henry Dreyfus Foundation (to B.A.G.). K.J.M.B. was supported by a NSF Graduate Fellowship.